Agricultural harvesters such as combines sever crop plants from the ground, thresh the crop plants, separate the grain portion of the crop plants from the remainder of the crop plants, deposit the remainder of the crop plants (MOG) on the ground, and store the grain in a grain reservoir or tank. Periodically, the grain in the grain reservoir is offloaded to a cart or truck that travels alongside the agricultural harvester and is carried away for storage.
There are several mechanisms inside the agricultural harvester that are used to separate the grain from the MOG. To ensure their proper settings and operation, the performance of various subsystems must be monitored. One way of monitoring the performance of these agricultural harvester subsystems is by determining how much grain passes through particular mechanisms within the agricultural harvester.
One way of monitoring the passage of grain is by use of a particulate matter impact sensor. These impact sensors are typically called “grain sensors” or “grain loss sensors”. These types of sensors are configured to sense the impact of a falling kernel of grain upon a surface of the sensor, and to convert this impact into an electrical signal. This raw electrical signal is then processed and transmitted to an ECU for further use.
This use can be as simple as displaying the amount (or relative amount) of grain impacting the sensor. With this information alone, an operator, based on his experience, can adjust the machine manually to improve performance. Alternatively, the use can be more complex, such as by employing the signal to automatically adjust the operating settings of various internal mechanisms of the agricultural harvester.
A common grain loss sensor design in agricultural harvesters comprises a flat impact plate, generally rectangular, to which a sensing element is attached on the rear side of the impact plate. The grain impacts the front side of the impact plate, causing the impact plate to flex.
A sensing element (typically a piezoelectric sensing element in the form of a thin layer) is attached to the back side of the plate. When the plate flexes, it causes the sensing element on the backside the plate to flex in a similar manner. The sensing element, in turn, is coupled to sensing circuits that receive the minute electrical signals from these flexures. The sensing circuits amplify and filter these signals, and convert them into a form that is usable by a digital microprocessor.
The common grain loss sensor design suffers from several defects. Some of these defects are due to the characteristics of the piezoelectric sensing element itself, and some of them are due to the inhomogeneity of the design overall.
Piezoelectric sensing elements mounted as described above, generate minute electrical signals based upon the gross bending of the sensor element rather than the localized bending at the point of impact of the kernel of grain upon the impact plate. The impact plate is typically made of relatively rigid material, such as fiber reinforced plastic, aluminum, or steel that is a few millimeters thick. The kernels of grain contact perhaps a 10 mm2 area of the impact plate. The impact plate, due to its stiffness, however, does not flex locally in response to the impact. Instead, each grain impact causes substantially the entire impact plate to bend inwardly an infinitesimal distance causing an extremely small curvature of the impact plate.
One example of a prior art arrangement can be seen in FIGS. 1-2, the dynamic response of which is discussed below.
In FIG. 1, a shallow metal housing or box 100 is fixed to an impact plate 102. The impact plate 102 has an outer surface 104 against which grains of crop, such as soybeans 106, corn kernels 108, and wheat 110 impact. It has an inside surface 112 which is bonded to a piezoelectric sensing element 114. A signal conditioning circuit 116 is fixed to the sensing element 114. A signal lead 118 is fixed to the signal conditioning circuit to provide it with electrical power and to receive back conditioned signals. These signals are received by an ECU/DSP for further processing and use.
The impact plate 102 is fixed at its edges to the edges of the shallow metal housing 100. It may be fixed with mechanical fasteners 120 such as rivets, bolts, screws, etc., or an adhesive 122. The edges of the impact plate 102 are therefore constrained in their movement by being fixed and/or coupled to the edges of the shallow metal housing 100.
The impact plate 102 is homogeneous in construction in that it has a constant thickness and constant material characteristics over substantially its entire extent.
The impact plate 102 is large compared to the size of the seeds that contact it. The impact plate 102 typically ranges from 75 mm×75 mm to 125 mm×250 mm.
The sensing element 114 does not extend over the entire inner surface of the impact plate 102. Grain can contact the impact plate 102 at any of locations 124, for example, that is disposed away from the sensing element 114. In order to communicate this impact to the sensing element 114 itself, the entire impact plate 102 must flex in response.
To illustrate this effect, an exaggerated view of this flexure in response is shown in FIGS. 3-5.
In FIG. 3, the seed 106 is approaching the impact plate 102 at a velocity “V”. The seed approaches the impact plate 102 at the center of the plate, equidistant from all the edges of the plate. The impact plate 102 has the sensing element 114 attached to its inner surface 112. The edges of the housing 100 that support the edges of the impact plate 102 are shown schematically as ground symbols. They are fixed and stationary.
In FIG. 4, the seed 106 has just contacted the impact plate 102. Given the deformation of the typical seed at a velocity “V”, the contact area is approximately 2 mm×2 mm.
In FIG. 5, the seed 106 has transferred all of its kinetic energy into the impact plate 102 and has slowed to a stop. The impact plate 102 has stored the kinetic energy of the seed 106 by elastic deformation. The impact plate 102 has flexed inward, becoming concave on its outer surface 104 and convex on its inner surface 112.
In FIG. 5, the impact plate 102 has flexed inward an amount “D”. This inward flexure of the impact plate 102 has caused the corresponding and substantially equal flexure of the sensing element 114. Since the sensing element 114 is disposed on the back of the impact plate 102, it is stretched in a direction “X” that is generally parallel to the plane of the sensing element 114. It is also flexed into a similar concave shape as the impact plate 102 since it is bonded to the inner surface 112 of the impact plate 102. This stretching and flexure of the sensing element 114 affects substantially the entire area of the sensing element 114.
In FIG. 6, the energy stored in the impact plate 102 and the sensing element 114 has been released and the seed 106 has been propelled in the reverse direction. The impact plate 102 has returned to its initial position (in this case, generally flat) as well as the sensing element 114. The electrical signal that was produced by the flexure of the sensing element 114 has disappeared since the sensing element 114 has returned to its initial, and stressed, shape.
In FIG. 7, the impact plate 102 and the sensing element 114 continues moving in the reverse direction until both have achieved a convex configuration. In order to get a fast response, the loss sensors are underdamped, which permits them to oscillate convex>concave>convex>concave as the energy input by the seed 106 dissipates. As this oscillation occurs, the signal from the sensing element 114 continues. This “ringing” of the sensor occurs at a relatively low natural frequency, often taking 10 or 15 ms to decay. In an overdamped sensor arrangement, the seed 106 would bounce off the (concave) outer surface 104 (FIG. 5) and the impact plate 102 and the sensing element 114 would gradually return to their planar position (as shown in FIG. 6) without achieving the convex position shown in FIG. 7. In this situation, the gradual restitution of the sensing element 114 to its initial planar shape would cause a gradual falloff of the signal produced by the sensing element 114. As in the case of the underdamped system, this gradual signal falloff can take 5 to 10 ms.
The 5 to 10 ms decay of the signal from the sensing element 114 has been determined to be a function of its overall size, mass, and stiffness.
The description above illustrates the ideal situation in which a seed 106 impacts the center of the impact plate 102 causing equal deflection of the impact plate 102 and the sensing element 114 in all directions. Given the symmetry in all directions about the center contact point of the seed 106 against the impact plate, the physical characteristics of the impact plate 102 and the sensing element 114 mandate that the response will be generally as shown in FIGS. 3-7.
In the real world, however, substantially the entire outer surface 104 of the impact plate 102 can be impacted by the seed 106. When an off-center impact by the seed 106 occurs, the characteristics of the resultant signal changes in unpredictable ways. An off-center impact of the seed 106 against the impact plate 102 is illustrated in FIGS. 8-10.
In FIG. 8, for example, a seed 106 approaches the impact plate 102 in a position that is off-center and adjacent to the edge of the impact plate 102. The impact plate 102 is supported on all sides by the housing 100. As in the examples of FIGS. 3-7, the housing 100 is represented as a ground symbol for convenience of illustration.
In FIG. 9, the seed 106 has contacted the impact plate 102 and has deflected it inward in the region surrounding the point of impact. Since the point of impact is off-center and immediately adjacent to the housing 100 which supports the impact plate 102, the movement of the impact plate is constrained. The impact plate 102 can no longer flex symmetrically across substantially its entire surface. Instead, as the energy of the seed 108 is absorbed by the impact plate 102, the impact plate 102 is deflected into a second mode of oscillation in which a portion 200 of the plate adjacent to the seed 106 is flexed into a concave shape, and a portion 202 of the impact plate 102 away from the seed 106 is flexed into a convex shape. Similarly, the sensing element 114, which is disposed in a center region of the impact plate 102, reproduces a similar convex/concave flexure.
The distance “D” of the concave flexure is smaller than the distance “D” of the concave flexure for a center impact (see FIG. 5) since the seed one impacted the impact plate 102 adjacent to fixed support (i.e. the edge of the impact plate 102 where it is fixed rigidly to the shallow housing 100). Since the distance “D” is reduced for a seed impact adjacent to an edge of the impact plate 102 as compared to a seed impact in the center of the impact plate 102, the sensing element 114 flexes much less, and therefore generates a much smaller electrical signal for impacts adjacent to the edge. This change in signal amplitude based upon the position of the seed impact on the surface of the impact plate 102 makes it difficult to properly condition the signal.
The signal problem is further complicated since the sensing element generates a signal related to its degree of stretching and its flexure. The portion 204 of the sensing element 114 adjacent to the seed and is under tension—it is stretched. However, the portion 206 of the sensing element 114 on the other side of the grain loss sensor is compressed. The signal produced by the sensing element 114 is an average of the tension/compression effects across the entire surface area of the sensing element 114. Since the sensing element 114 is experiencing both a tension in one portion 204 and a compression in another portion 206, the electrical signal generated by the sensing element 114 overall is even further reduced, since these two areas generate opposing electrical signals that (in effect) cancel each other out.
FIG. 10 illustrates the oscillation of the impact plate 102 when the impact plate 102 plus sensing element 114 combination is underdamped after the seed 106 is released. In a typical grain loss sensor of this design, the impact plate 102 can oscillate back-and-forth between the two extreme positions illustrated in FIG. 10. In this case, the impact plate 102 plus sensing element 114 will gradually return to the position shown in FIG. 8 as their energy is dissipated. This can take 5 to 10 ms.
Alternatively, if the impact plate 102 plus sensing element 114 combination is overdamped, it will release the seed 106 in the position shown in FIG. 9 and relax to the generally planar position shown in FIG. 8. Depending on the degree of over damping, this gradual return to the position shown in FIG. 9 can take 5 to 10 ms.
A further complication is the difference in vibrational frequencies generated by the seed one impact of FIGS. 3-7 and the seed 106 impact of FIGS. 8-10. When the impact plate 102 oscillates in its primary mode (shown in FIGS. 3-7) it has a frequency of oscillation that is less than the frequency of oscillation caused by the off-center seed impact shown in FIGS. 8-10. This also adds to the complexity and difficulty of determining individual seed impacts.
In the description above regarding FIGS. 3-10, I have illustrated only two different positions at which the seed 106 may impact the impact plate 102. There are an infinite number of positions at which a seed one can impact the impact plate 102. Further, the wave equation predicts that there are an infinite number of modes of oscillation that can be generated by each impact, each mode of oscillation having its own distinct (and different) frequency and its own distinct (and different) amplitude.
The final complication in determining seed impacts on a grain loss sensor of this design is when several seeds make contact the impact plate at many different locations on the impact plate within milliseconds of each other. Given the long decay time from a single seed impact (5-10 ms), it is virtually impossible to distinguish individual seed impacts when they are occurring faster than once every 20-30 ms or so. There are so many possible modes of oscillation (based upon the location of the strike), there are so many possible amplitudes (based upon the location of the strike) and there is such a long decay time (because of the large mass of the sensing element 114 and the impact plate 102) that identifying and quantifying grain strikes with any accuracy is virtually impossible.
One potential solution is to provide many more of these grain loss sensors and arranging them side-by-side, each of these grain loss sensors having a smaller impact plate. The costs of doing this are high, however.
Another potential solution is to have a single large impact plate 102, but to bond multiple, smaller sensing elements 114 in an array on the back of the impact plate 102. This, too, would be quite costly. The number of electrical interconnections that would have to be made to each individual sensor would be prohibitive. Further, each sensor would still generate a signal that was a composite of the effects of grain strikes all over the sensor plate since the oscillations of the impact plate 102 would still be communicated through the impact plate from the point of impact to other adjacent regions of the impact plate 102. Separating out spurious components of the signal from one sensing element 114 due to grain strikes on a distant portion of the impact plate 102 would be virtually impossible. Finally, as the number of the sensing elements 114 bonded to the back of the impact plate 102 is increased, the size of each sensing element 114 would have to be correspondingly reduced. The reduced size of each sensing element 114 would cause a corresponding reduction in signal amplitude which would thus make each sensing element 114 much less sensitive and much more subject to electrical and mechanical noise.
What is needed, therefore, is a sensor arrangement that is faster, that has less noise, that has a higher frequency of response, and/or has a lower dynamic mass. It is an object of this invention to provide such a sensor arrangement.